Development of a Biodegradable Material with Oregano Stick as a Prototype of Substitute for Wooden Agglomerate Material

Abstract

Oregano is a herb that is found in the wild in different parts of the world. The stick represents about 60% of the plant and is biodegradable, using lactic acid as a binding agent for the oregano stick. Waste oregano stick can be used to make biodegradable material which provides sustainable development in support and promotion of the circular economy by reducing pollution generated by chemical economy by reducing pollution generated by chemical products, agricultural waste, or products that accumulate in the environment spending years for its degradability. The originality of this project is present in the use of the oregano stick, a natural product which supplies the physicochemical characteristics of conventional raw materials used in the manufacture of wood. Oregano was collected and dried to separate the leaves from the stick. The stick was then ground, sieved at 0.118 mm and 0.025 mm, and then treated with one of two binders. For treatment 1, three samples were prepared with varying lactic acid concentrations and stick quantities with the addition of 0.3 mL of 2,2 azobisiso-butyronitrile (AIBN) 1% by weight. In treatment 2, three samples were prepared with varying polyvinyl acetate concentrations, diluted in acetone, stirred, poured into a mold, and placed in an oven at a temperature of 90 °C for 36 h. Bending and compression mechanical tests were performed on all samples; the data were analyzed by one-factor analysis of variance and Tukey test. The sample that performed the best had a value of 1.148 mm and 0.77333 kN in mechanical resistance and less elongation in compression. This sample also had a value of 0.8183 and a kN of 0.1559 with a lower mean, meaning higher resistance to bending. This sample also had the best results from the Modulus of Rupture (MOR) test with a value of 5.9958 MPas/m².

1. Introduction

Oregano is a wild plant located in arid and semi-arid areas worldwide, mainly represented by two species: Origanum vulgare, native to Europe, and Lippia graveolens, native to North and Central America. Although its cultivation is still traditional and limited, the development of agricultural activities around oregano crops has increased in recent years in some countries, due to its growing demand. The sticks are rigid, hairy, and aromatic, and its extractives are made up of tannic substances [1]. It is a shrub that has different nutraceutical properties, one of which is antioxidant activity that can limit or inhibit the oxidation of biomolecules, can promote antiseptic, antiviral, and anti-inflammatory activity, and provides health benefits for those with chronic degenerative and neurodegenerative diseases [2]. Oregano crops require adequate agronomic management to improve production, including irrigation, pruning, transplanting, and other activities that allow for the optimization of the crop. In this process, part of the plant is generally discarded, becoming organic waste [2]. As expressed by Pérez et al. [1], the cultivation of oregano generates an approximate production of 1650 kg of dry leaves and 2475 kg of sticks and branches per hectare. One important characteristic of oregano is that the stick from oregano from desert areas has a high tannin content and, if the quality is good, can be a source of raw material for the extraction of phenolic compounds [3]. When harvested from the wild, the amount of stick varies greatly and is largely dependent on the species and environmental factors such as climate, soil, and humidity [4].

In Mexico, around 4000 tons/year of leaves of the species Lippia graveolens are produced, generating a considerable amount of stick and branches that are not used [4]. Producers usually burn this waste or throw it away without obtaining a benefit. Using these sticks and branches is a viable option since the amount of waste generated is more than 60% of the plant [5].

Some species of herbs, such as oregano, have antimicrobial properties, containing compounds that are both simple and complex derivatives of phenol and are volatile at ambient temperature [6]. Oregano leaves are added to food as flavoring agents. It has been known since ancient times that oregano and its essential oils have varying degrees of antimicrobial activity. The leaves of the plants more efficiently store more chemicals in the form of secondary metabolites, with microbial activity. Research into the antimicrobial action of oregano oil justifies its use in the preparation and preservation of food in addition to medicinal use. The variation in its constituents can interfere with that microbial activity. In Latin America, oregano is used as a medicinal herb due to its stimulating diaphoretic properties. It is used as a nerve tonic, indigestion relief, and emmenagogue (promoter of menstrual flow) in addition to being used to treat insect bites, headaches, toothaches, and some types of coughs. Oregano essential oils also have fungicidal and antiparasitic properties [5,6,7].

The agglomerated material is manufactured mainly by wood pellets, representing a method for the spread of pests, being made of wood chips and resin, which, by containing adequate moisture, tends to lose its mechanical resistance. However, wood with a moisture content greater than 20% is prone to being attacked by fungi. Other methods of obtaining the agglomerate are cardboard, which is inexpensive, recyclable, and easy to handle, and plastic, which is reusable and highly impermeable and is available in a wide variety of forms [8,9].

The use of biodegradable, biocompatible, and environmentally friendly materials such as lactic acid is an alternative for binding the oregano stick that could reduce pollution and replace the use of plastics [6]. According to Duran [7], reducing the loss of raw material and optimizing production occupies a fundamental place in the productive sector. As a consequence, emphasis has been placed on making better decisions regarding the use of the available raw material by appreciating its mechanical properties to make this type of compound [8]. Just as wood residues in the form of shavings and sawdust are used in the development of particulate panels, it can take advantage of mechanical properties of the oregano stick to make this type of compound.

The objective of this study is to develop a substitute prototype for wood material through the use of oregano sticks. The study uses mixtures of two binders and a polymerization initiator, generating a pallet on which mechanical and Raster Electron Microscopy (REM) analysis tests were subsequently conducted.

Oregano sticks as a waste residue can be reused to make products such as pallets. When using the oregano stick in the highest percentage possible, environmental impact from the manufacture of these products can be reduced since the main conventional source of the raw material are trees that have a long biological cycle. Oregano is a plant with a short biological cycle, and the stick is a good quality raw material that can be potentially used to make pallets.

2. Materials and Methods

To conduct this study, a five-stage methodology was proposed, including preparation of the material and analysis of the results.

2.1. Preparation of Oregano Raw Material

This research took place at the experimental fields of the National Technological Institute of Mexico and Guadiana Valley Technological Institute, located in Durango, Mexico. The oregano raw material was collected and left to rest for two weeks in the absence of light for drying. Afterwards, the leaves were fractionated from the stick, and impurities were eliminated. The stick was then crushed with the help of a mill manual (mortar and pestle) and sieved at 0.118 mm and 0.025 mm.

2.2. Preparation of Treatments

The experimental design was based on two treatments. For the first treatment, lactic acid was applied as a binder and in the second treatment, polyvinyl acetate was applied. A total of six samples were prepared: three samples (A, B, and C) with treatment one and three (D, E, and F) with treatment two. Three replicates were performed per sample and (

Table 1. Experimental design for each of the samples.

Sample	Sieve (mm)	Quantity of Soaked Stick (g)	Binding Agent	Amount of Binding Agent (mL)	2,2 Azobisisobutyronitrile (AIBN) (mL)
A	0.025	100.0	Lactic acid	30	0.30
B	0.025	125.0	Lactic acid	40	0.40
C	0.025	110.6	Lactic acid	44	0.44
D	0.118	92.0	Polyvinyl acetate	20	0.18
E	0.118	92.0	Polyvinyl acetate	35	0.30
F	0.118	92.0	Polyvinyl acetate	42	0.40

) shows the quantities used for each of the samples.

The study used 700 g of oregano stick of the L. graveolens variety, which was divided into two groups: the first with lactic acid and the second with polyvinyl acetate, divided into 12 prototypes of 270 g each with a size 15 cm × 5 cm × 1.2 cm, which were subjected to mechanical compression and flexion tests (

Table 2. Results of the applied treatments.

Sample	Binding Agent	Compression Test		Flexion Test
		mm	kN	mm	kN
A (1A)	Lactic acid	3.9814 ± 0.32	0.8513 ± 0.032	0.3542 ± 0.003	0.0540 ± 0.013
B (2A)	Lactic acid	2.8578 ± 0.28	0.4197 ± 0.015	0.8427 ± 0.030	0.0420 ± 0.011
C (3A)	Lactic acid	1.9663 ± 0.38	0.4676 ± 0.015	0.0122 ± 0.001	0.0540 ± 0.013
D (1B)	Polyvinyl acetate	1.9451 ± 0.32	0.6295 ± 0.022	2.0640 ± 0.023	0.1499 ± 0.023
E (2B)	Polyvinyl acetate	1.8686 ± 0.30	0.8273 ± 0.032	1.2946 ± 0.020	0.0420 ± 0.011
F (3B)	Polyvinyl acetate	1.1480 ± 0.21	0.7734 ± 0.028	0.8183 ± 0.031	0.1559 ± 0.028

).

2.2.1. Lactic Acid Treatment (A, B and C)

For this treatment, oregano sticks weighing 50 g were sieved at 0.025 mm and placed in a beaker with 200 mL of water to soak. The soaked oregano stick is reweighed and shaken in a beaker at 90 rpm. The lactic acid is then added with the AIBN (1% by weight) and stirred for 5 min; the amounts of the reagents varied according to the treatment (Table 1).

2.2.2. Polyvinyl Acetate Treatment (D, E and F)

For this treatment, the polyvinyl acetate solution was prepared mixing 100 mL of acetone in a beaker, adding 30 g of polyvinyl acetate, and letting it sit for 2 h, after which the solution was shaken to obtain a homogeneous mixture.

As in the previous treatment, 50 g of oregano stick (sieved at 0.118 mm) was weighed, and 150 mL of water was added for soaking. The soaked oregano stick was weighed and shaken in a beaker at a speed of 90 rpm. The polyvinyl acetate and AIBN were poured into a beaker and shaken for 1 min. Finally, the soaked oregano stick was added and stirred for 30 s until a homogeneous mixture was obtained.

2.3. Drying Process

The oregano stick paste was then poured into a mold and placed in the drying oven at 90 °C for an estimated 36 h. Three pieces of 5 cm width by 20 cm length and 1.5 cm height were designed with different concentrations of polyvinyl acetate and lactic acid to be later evaluated by physical analysis of compression and flexion.

2.4. Mechanical Bendign and Compression Tests

Using a universal testing machine mechanical bending and compression tests were performed. Secondly, the MOR index (Modulus of Rupture) was calculated (Equation (1)).

The factor Fu is the load at rupture in newtons, the factor L is the length between supports, the factor L₂ is the width of test tube and the factor L₁ is the thickness of the test tube.

$$MOR=\frac{3 Fu L}{2 L_2 L_1^2}$$

(1)

2.5. Statistical Analysis

A one-factor Analysis of Variance (ANOVA) test and a Tukey’s test [9]; were performed on the data. The data were processed in the SPSS version 10 statistical program [10].

3. Results

The two treatments used different binders, one of which used polyvinyl acetate at three different concentrations. In sample 1B, 20 mL of acetate, 0.18 mL of AIBN, and 92 g of soaked oregano stick (0.118 mm) were used. In sample 2B, 35 mL of acetate, 0.3 mL of AIBN, and 92 g of soaked oregano stick (0.118 mm) were used. In sample 3B, 42 mL of acetate, 0.4 mL of AIBN, and 92 g of soaked oregano stick (0.118 mm) were used. The acetate samples had a good appearance but were somewhat rough in texture (Figure 1). Modules of Rupture was 5.99 Mpa/m².

The three concentrations of lactic acid are shown in the samples. In sample 1A, 30 mL of lactic acid, 0.3 mL of AIBN, and 100 g of soaked oregano stick (0.025 mm) were used. In sample 2A, 40 mL of lactic acid, 0.4 mL of AIBN, and 125 g of soaked oregano stick (0.025 mm) were used. In sample 3A, 44 mL of lactic acid, 0.44 mL of AIBN, and 110.6 g of soaked oregano stick (0.025 mm) were used. The lactic acid samples had a less rough appearance than the acetate sample with small cracks on the surface. When changing color, the lactic acid samples changed to a darker color than the acetate samples (Figure 2). In hardness, the lactic acid samples were very similar to acetate in the highest concentrations.

The sample with the lowest mean is 3B with an elongation of 1.1480 mm and a resistance of 0.7734 kN. Secondly, the sample with the highest strength is sample 1A with 0.8513 kN in the compression test (Table 2).

The results of the analysis of variances of the compression and flexion tests are presented below. The following hypotheses are taken:

• H₀ (null or equality hypothesis): the means of the samples are equal.
• H₁ (alternative or difference hypothesis): the means of the samples are significantly different.

3.1. ANOVA and Tukey’s Range Test for Compression Test

The ANOVA analysis of variance uses two hypotheses when checking if the response variable gives us a value greater than 0.05 or less than 0.05. If the result is greater than 0.05, the null or equality hypothesis is accepted, but if the value obtained is lower, the null hypothesis is rejected, and the alternative hypothesis is accepted. In the study results, a value p = 0.000 was obtained, meaning that p < 0.05 and leading to the conclusion that there is a significant difference among the factors compared (

Table 3. ANOVA for the compression test.

Source	GL	SC Adjusted	MC Adjusted	F Value	p Value
Sample N°	5	34.55	6.9099	10.38	0.000
Error	316	210.29	0.6655
Total	321	244.84

). For the compression test, a 5 inc specimen is used that is 5 cm × 5 cm, similar to that used in the flexion test with speed of 5 mm/min, according to ASTM 1037 [11].

The results from the Tukey’s test suggest that samples A and F are very different from the other samples (

Table 4. Tukey’s range test for the compression test.

Sample N°	N	Average	Group
A (1A)	69	1.8740	a
B (2A)	72	1.3580		b
C (3A)	45	1.2045		b	c
D (1B)	50	1.1864		b	c
E (2B)	56	1.1766		b	c
F (3B)	30	0.7242			c

). The sample that has the lowest mean is F, making it the sample that has the highest resistance and least elongation in the compression test. Conversely, the A sample has the highest mean showing that it is the sample with the least resistance and greatest elongation in the compression test.

Turkey’s test indicates that the samples exhibit significant differences. The information on the distance factor shows that sample 3B 4 was the one with the lowest elongation, which has the highest resistance as it has the lowest distance with value of 0.75 mm in the compression test. It is also observed that the sample that obtained the highest value and therefore the lowest resistance to elongation was the 2A m sample with an elongation of almost 2 mm in the compression test. The graph shows that there are significant differences with respect to each of the samples. Samples 1B e, 2B a, and 3A a almost coincide in the elongation distance, so they are the most similar, although the binder is different and the values that show in these three samples have a minimum difference with respect to the samples 3B 4 and 2A m that were the ones that obtained the maximum value and the minimun value (Figure 3).

The percentage of elongation that was obtained in the compression test on each of the samples (Figure 4).

The highest value is found in the 2A m sample with more than 6% and the lowest, the sample 3B 4, with over 2% elongation. Samples 2B a, 3A a, and 1B e present similar values being the samples that obtained a percentage of around 4%, the 2A t sample was the one that came closest to the maximum value of elongation with a percentage above 4%.

Figure 5 shows six curves; the green curve corresponds to the acetate sample with the highest concentration which presents a resistance that is higher and very significant compared to the gray curve whose concentration of lactic acid was lower. The values of the blue, orange, and gray curves indicate a greater elongation at less pressure, having less resistance to compression. The results show that acetate has greater hardening properties than lactic acid, although lactic acid samples have a higher tenacity with higher flexion points than the polyvinyl acetate samples.

3.2. ANOVA and Tukey’s Range Test for Flexion Test

The results show a value p = 0.000, meaning that p < 0.05, which leads to the conclusion that there is a significant difference between the factors compared (

Table 5. ANOVA for the flexion test.

Source	GL	SC Adjusted	MC Adjusted	F Value	p Value
Sample N°	5	98.32	19.6634	20.55	0.000
Error	316	302.36	0.9568
Total	321	400.68

). For the bending test, the chipboard was cut to the dimensions of 20 cm × 5 cm according to ASTM 1037 standards for thicknesses of agglomerates smaller than 6 mm. The test was performed with the universal machine at a speed of 5 mm/min [11].

The flexion test indicates that sample F had the shortest flexion distance with an average of 0.6005 mm, meaning this sample had the most resistance to flexion. Sample A had the highest value with an average of 2.173 mm, which indicates that it was the least resistant to bending (Figure 6).

Observing the differences that exist between the samples that have the highest values in the flexion test, sample C most approached the maximum resistance value and sample B most approached the minimum resistance value (

Table 6. Turkey’s range test for the flexion test.

Sample N°	N	Average	Group
A (1A)	69	2.1730	a
B (2A)	72	1.9720	a
C (3A)	45	0.8194		b	c
D (1B)	50	1.2843		b
E (2B)	56	1.8450	a
F (3B)	30	0.6005			c

).

Samples 2A 1 and 1B present a greater deformation, which means that they have greater tenacity. Sample 3B 1 presents a similar value.

Both polyvinyl acetate and lactic acid samples have little deformation in comparison with samples of other materials such as Merremia tuberosa in which a greater slope is observed in the elastic zone. In addition, M. tuberosa has a greater plastic zone due to its deformation that can reach around 6 mm without breaking. The oregano stick samples show a lower slope in the elastic zone, with a deformation achieved around 2.17 mm (Figure 7). This result confirms that the toughness of the agglomerates has minimal variations due to the binder used and depends more on the wood used.

The oregano stick samples that used lactic acid as a binder in the bending test showed the highest tenacity. However, their values are far removed from the M. tuberosa samples, using polyvinyl acetate as a binder that reaches 6 mm, while those of oregano stick do not reaches neither the mm applying a smaller force.

3.3. Modulus of Rupture (MOR)

The modulus of rupture (MOR) according to the international standard ISO 10545-4 [12] is an intrinsic characteristic of the material. In other words, chipboards manufactured using the same process that differ only in thickness will have the same MOR, although the force required to break them will be much greater in the thicker chipboard [13].

The oregano stick agglomerate presents lower MOR values than the commercial agglomerate (

Table 7. Mechanical properties of oregano (PO) and commercial (AC) wood agglomerates.

Source	L (m)	L1 (m)	Fu (N)	MOR (MPas/m2)
Oregano stick agglomerate	0.201	0.012	143.9	5.9958
Commercial agglomerate	0.309	0.053	25343.3	13.4247

). However, the oregano stick agglomerate is four times thinner than the commercial agglomerate and does not undergo the pressing process. The commercial agglomerate does undergo the pressing process, which decreases its resistance to bending.

4. Discussion

The development of new materials from waste is a practice that is becoming more common nowadays. The idea behind this practice is to use generated waste raw material to create new, useful, and sustainable materials. Organic waste can be used to create bioplastics, which are plastics made from biological materials, such as oregano sticks [14].

Bioplastics production is a process that involves creating plastics from biological materials, such as corn starch, cellulose, lignin, and other organic compounds [15]. Bioplastics are used to create a wide variety of products, such as bags, containers, bottles, and toys. The advantage of bioplastics is that they are biodegradable unlike conventional plastics that take hundreds of years to degrade and release pollutants into the environment [16]. However, it should be noted that the production of bioplastics still faces challenges, such as limited availability of raw material and efficiency of the production process. As technology advances, solutions to these challenges may be found, and the production of bioplastics may become more sustainable and profitable [17].

Based on the results, it is evident that the oregano wood samples have a compressive strength of 20 ton/m², while the commercial agglomerate has a strength of 66.28 ton/m² [13]. This difference is due to the fact that the commercial agglomerate is 5 cm thick while the oregano stick samples are 1.2 cm thick. Another difference between the commercial agglomerate and the oregano stick samples is that the commercial agglomerate generally undergoes a pressing process and the oregano stick samples in this study did not.

The folding methods used to obtain the agglomerated material represent an alternative to improve the characteristics and properties of the final product by giving the material elasticity and flexibility, endowment of solidity due to the execution of force and resistance to the material, prevention of damage and deterioration, and high resistance and adaptability to the application or the industry in which the obtained product is implemented.

The use of biodegradable and environmentally friendly binders such as lactic acid and polyvinyl acetate for the production of pallets made from oregano sticks is a good option to reduce the use of polluting and carcinogenic chemicals, such as urea or formaldehyde, which is the most widely used adhesive for wood particles. The use of the oregano sticks to make pallets will reduce CO₂ emissions into the air because the producers will sell the material instead of burning it as they regularly do to get rid of the residues [13]. In addition, they could potentially benefit economically from the sale of the oregano stick.

Annual plant wastes such as flax, hemp, jute stalks, bagasse, cotton stalks, walnut shells, rice husks, and palm stalks are cheap and valuable raw materials for the production of lignocellulosic boards. MDF boards contain 82% wood fibers or annual plants, 9% amino resin sizing, 1% paraffin, and 8% water [18]. According to Taj et al. [19], certain proportions of binder, stick, and water were used in the oregano stick agglomerate because there are differences in the manufacturing process compared to commercial MDF. In commercial MDF, the material is hot pressed which increases the adhesion of particles with less binder but increases their density.

According to Azwa et al. [20], natural fibers have been used over the last 3000 years to reinforce materials; more recently, natural fibers are being used in combination with plastics. Research has been conducted on a wide variety of natural fibers due to their advantage as renewable resources, being a marketing attraction, just as the present investigation is to give a reuse to organic waste in order to lower the rate of waste generation, citing Cordero-Villa et al. [21] the residues generated after the extraction of the oregano essential oil (stick and leaf), took advantage of it for the production of composts; since in the region of Rodeo, Durango, this waste is eliminated through burning it and pollution of the environment is generated.

According to López and Rodríguez [22], natural fibers have been used for many years in Asian markets. One example is jute, very commonly used as reinforcement in India. Today the use of natural fibers in the automotive industry and packaging materials has increased.

Natural fibers have also gained the attention of researchers for their use as reinforcement in polymeric composites (NFPCs) and their applications [23]. The effects of various chemical treatments, such as thermosetting and thermoplastic reinforcement, have been favorable for the chemical and thermal properties of natural fibers. The chemical treatment of natural fibers improves the adhesion between the fiber surface and the polymer matrix, optimizing physical-mechanical and thermo-chemical properties [24].

According to Rodriguez et al. [25], M. tuberosa is a plant with a short biological cycle that has been the subject of research for the elaboration of agglomerates. Results show significant mechanical properties, making it a potential alternative as reinforcement in polymeric matrices. Due to its low volumetric density, the plant could be used as a sandwich type chipboard core to reduce weight and increase elasticity of the product [26,27,28].

5. Conclusions

The raw material used to produce bioplastics from oregano stick waste is lignin, a polymer found in the cell wall of many plants. Lignin is a complex organic compound that can be broken down into its constituent monomers through a process of hydrolysis. The resulting monomers can be used to produce a wide range of bioplastics, including thermosetting and thermoplastic polymers. The use of biodegradable and environmentally friendly binders such as lactic acid and polyvinyl acetate for the production of pallets made from oregano sticks has the potential to reduce the use of polluting and carcinogenic chemicals, including urea formaldehyde, which is widely used as an adhesive for wood particles. The commercial agglomerate presents a resistance 331% higher than that of the oregano stick, partially due to the differences in thickness; the commercial agglomerate is nearly four times thicker than the oregano stick agglomerate.

The production of bioplastics from oregano stick waste offers several advantages compared to the production of bioplastics from other raw material sources. Firstly, oregano stick is a waste that is produced in vast quantities around the world, which means that its use in the production of bioplastics has the potential to significantly reduce the amount of waste sent to landfills. In addition, the production of bioplastics from oregano waste is more sustainable than the production of conventional petroleum-based plastics because the production of bioplastics emits less greenhouse gases and reduces reliance on fossil resources. The production of bioplastics from oregano stick waste is an emerging technology with great potential to contribute to environmental sustainability and reduce the amount of waste sent to landfills. As technology advances and associated technical challenges are resolved, we are likely to see an increase in the production and use of bioplastics from oregano stick waste.